The degree of pigmentation of the rind of apples is one of the very important quality requirements and it determines their market value. Improvement in the degree of pigmentation of apple rind has been an important subject for apple growers.
While the mechanism of action in pigmentation of apple rind has not yet been elucidated completely, it is known that sufficient exposure to sunlight leads to an improvement in pigmentation. In order to provide more sunlight, a practice widely followed is to lay a silvery sheet under the trees or to thin out the foliage. However, these methods of increasing the amount of sunlight exposure not only cost the growers a large amount of labor but produce insufficient effects. In addition, thinning the foliage decreases the vigor of the plant.
On the other hand, an approach for improving pigmentation of apple rind by using a chemical agent has also been made. However, a number of chemically synthesized agricultural chemicals, including agents for improving pigmentation of apples, have been revealed to have high toxicity or to remain and accumulate in the environment. For example, recently a permission to "Alar" (SADH) developed for that purpose was revoked on suspicion of carcinogenicity. Therefore, development of naturally occurring substances which have low toxicity and do not remain in the environment has attracted attention as a new direction to be taken in agricultural chemistry.